The present invention relates to processing for the production of a reduced picture of a binary picture.
In present G4 facsimile 200, 240, 300 and 400 ppi (Pels/inch) are used as standardized resolution for paper sizes of ISO A4, ISO B4 and ISO A3. Pictures of such various paper sizes and such various resolutions are now employed in facsimile, and the conversion of resolution (a picture reducing system) is requisite for the intercommunication of pictures of different paper sizes and resolutions. A variety of picture reducing systems have been proposed. Typical systems are such as follows:
(1) SPC method (Gobo and Kirihara: One system for conversion of facsimile line density, Processing of National Conference of the Institute of Image Electronics Engineers of Japan, Vol. 7, No. 1, (1978))
This is a system which uses that one of picture elements on the original picture which is the closest to the picture element on a reduced picture.
(2) Projection method (Morita, Komachi and Yasuda: System for high-speed conversion of picture element density based on projection method, The Journal of the Institute of Image Engineers of Japan, Vol. 11, No. 2, (1982), for example) This is a system in which the mean density of original picture elements projected onto picture elements of a reduced picture is obtained and the value is processed using a threshold value to obtain the value of each picture element.
A system that has been proposed to retain thin lines is a TP method (Wakabayashi, Kawanishi and Adachi: Reduction and conversion method which prevents erasure of thin lines, The Transactions of the Institute of Electronics, Information and Communication Engineers of Japan, Vol. J70-D, No. 4 (1989. 7), for example), but this method is directed to the retention of vertical and lateral lines and does not take oblique lines into account. Thus, the conventional picture reducing systems are poor in the retention of thin line, and hence suffer from serious deterioration of picture quality.